HEREDITY is to-day the central problem of biology. This problem may be approached from many sides—that of the observer, the statistician, the practical breeder, the experimenter, the embryologist, the cytologist—but these different aspects of the subject may be reduced to three general methods of study, (1) the observational and statistical, (2) the experimental, (3) the cytological and embryological. Before taking up these different aspects of heredity it is important that we should have clear definitions of the terms employed and a fairly accurate conception of the processes involved.

1. Definitions

Heredity originally meant heirship, or the transmission of property from parents to children, and in the field of biology it has been defined erroneously as "the transmission of qualities or characteristics, mental or physical, from parents to offspring" (Century Dictionary). The colloquial meaning of the word has led to much confusion in biology, for it carries with it the idea of the transmission from one generation to the next of ownership in property. A son may inherit a house from his father and a farm from his mother, the house and farm remaining the same though the ownership has passed from parents to son. And when it is said that a son inherits his stature from his father and his complexion from his mother, the stature and complexion are usually thought of only in their developed condition, while the great fact of development is temporarily forgotten. Of course there are no "qualities" or "characteristics" which are "transmitted" as such from one generation to the next. Such terms are not without fault when used

↑Second of the Norman W. Harris Lectures for 1914 at Northwestern University on "Heredity and Environment in the Development of Men," to be published by the Princeton University Press.